It is a posture with which Barack Obama must be painfully familiar after 20 debates with Hillary Clinton: The Stare. Yesterday, it was John McCain's turn to feel those blue eyes boring into him as he spoke up manfully for a deeply unpopular war in Iraq. The senator from Arizona did not last the morning.

"Today it is possible to talk with real hope and optimism about the future of Iraq and the outcome of our efforts there," he said in his opening remarks to the Senate armed services committee.

Clinton, seated at the lower end of the crescent-shaped table, propped her chin up on her hand, the better to hold that unwavering gaze.

McCain uttered the word "success" four times in quick succession. It was not possible to tell from the distance of the press tables whether Clinton rolled her eyes before breaking her gaze. But she pursed her lips, looked down and took furious notes.

Officially, the interplay between McCain and Clinton in the morning and Obama's star turn in the afternoon were the sideshow to the testimony to Congress of America's commander in Iraq, General David Petraeus, and the ambassador to Baghdad, Ryan Crocker, on the gains under last year's troop surge.

But with all three senators hoping to follow George Bush into the White House, yesterday's sessions of the armed services and foreign affairs committees were an occasion to show off their defence credentials.

In the first Democratic debates, Obama seemed visibly uncomfortable with being the object of Clinton's gaze, and avoided eye contact. But there were nine ageing white males between McCain and Clinton, which perhaps diluted its power.

The only gap in McCain's presidential persona came when he was interrupted by a shouting anti-war protester, and let out a strangely adolescent chuckle.

McCain painted himself as the only real patriot in the race. He understood the importance of success in Iraq. "This means rejecting, as we did in 2007, the calls for a reckless and irresponsible withdrawal of our forces at the moment when they are succeeding," he said - the implication being of course that America would not be safe under Clinton.

Clinton, holding the reading glasses she sported in her TV advertisements about her suitability to take the 3am calls at the White House, did not blink. "I think it would be fair to say it might well be irresponsible to continue a policy that has not produced results," she said. Had McCain been in the room, it would have amounted to a full-scale onslaught on his Iraq policy. But as the senior Republican on the committee, he was allowed to speak well before Clinton. By the time her turn came, he was long gone.

For Obama, the clash between Senate protocol and the presidential race was especially wearing. Despite dominating the Democratic race for the White House he is a lowly number 13 on the pecking order of the Senate foreign relations committee. As he waited for his turn to question Petraeus and Crocker Obama's impatience - or possibly boredom - was palpable. He leaned back in his chair, he rocked, but his turn did come.

The senator was blunt: was there any measure of success under which they would recommend a withdrawal of US forces from Iraq? "If we were able to have the status quo in Iraq right now without US troops, would that be a definition of success?" Obama asked. "If we had the current status quo and yet our troops had been drawn to 30,000 would we consider that a success?"